Laser technology is applied for a large variety of material processing tasks, such as cutting, drilling, welding, marking, engraving and material ablation. Nearly all materials can be processed, e.g. metals, metal alloys, ceramics, diamonds, synthetic diamonds, carbon fibres, sapphire, quartz, glass, plastics and more. In almost every case, the laser light is focused into a very small spot onto the work piece using a focusing lens, to enable the processing task by generating sufficient energy on the work piece. The work piece therefore has to be precisely aligned into the laser focus throughout the processing task.
Liquid-jet guided laser technology, as for example described in patent EP 1940579B1 and U.S. Pat. No. 8,859,988B1, couples the laser focus into a small liquid-jet, for example, through a focusing lens. This coupling takes place in a coupling unit. The coupling unit can include a metal chamber that on the side of the focusing lens is closed with a laser protection window. On the opposite side the chamber carries a nozzle. Liquid provided to the coupling unit flows between window and nozzle and leaves the nozzle in form of a liquid-jet. The energy of the laser spot in the focal plane is captured inside the liquid-jet and guided to the work piece through internal reflection. This method eliminates the necessity to control the distance of the work piece precisely because the required energy to perform the processing is available throughout the laminar length of the liquid-jet. Any liquid that provides suitable light guide capabilities can be used to form the liquid-jet.
To increase the laminar length of the liquid-jet, and with that the working distance of the process, an assist gas can be provided to the liquid-jet as described in patent EP 1940579B1 or patent EP 1833636B1. The assist gas is provided and guided as a direct boundary layer to the liquid-jet in order to reduce the resistance between liquid and ambient air and thereby increase the laminar length of the liquid-jet. Thus the liquid jet is surrounded by the assist gas to leave the coupling unit through the same exit opening. Inside the coupling unit, the assist gas is directed perpendicular to the liquid jet. For example, the assist gas is in the horizontal plane hitting the liquid-jet that is travelling in the vertical plane. The assist gas and the liquid jet then leave the system at a same exit hole, with the liquid jet in the middle surrounded by the assist gas.
FIG. 1 illustrates a prior art liquid jet guided laser beam system having an assist gas configuration. A liquid jet guided laser beam system 100 can include a coupling unit 130 having a nozzle 135. A liquid, such as water 120, can be provided to the nozzle 135, and travel through the hole of the coupling unit to form a liquid jet 140 in a chamber 160. A laser beam 110 can be focused, for example, by a lens 115, to the liquid jet 140. Internal reflection can limit the laser beam to be within the liquid jet. The liquid jet guided laser beam can flow toward an object surface 190, where the laser can cut through the object by means of material ablation in a single or multiple passes.
An assist gas 150 can be provided to a cavity 160 of the coupling unit 130. The assist gas 150 can flow 155 in a direction perpendicular to the liquid jet 140, but do not intersect the liquid jet. The assist gas 155 can envelop the liquid jet, reducing the friction of the liquid jet to the air ambient, and potentially extending the laminar length of the liquid jet. The properties of the assist gas can be chosen to optimize the laminar length of the liquid jet, such as low viscosity gas at medium pressure.
Since the assist gas 150 and the liquid jet 140 are mixed in a same chamber 160, there is dependency between the assist gas and the liquid jet. For example, the pressure and flow properties of the assist gas can be selected to optimize the laminar flow of the liquid jet. Other operating conditions of the assist gas can adversely affect the liquid jet. For example, a high pressure of the assist gas can shorten the laminar flow of the liquid jet, and an even higher pressure of the assist gas can destroy the liquid jet.
There is a need for improving the liquid-jet laser technology, for example, to keep a work piece surface free from liquid accumulation while performing liquid-jet guided laser based material processing.